The present invention relates to a semiconductor device including a capacitor device having a capacitor dielectric film of an oxide dielectric film such as a ferroelectric film and a high dielectric film, and a method for fabricating the semiconductor device.
In a recently accelerated trend in processing and storing massive data resulting from development of digital technology, electronic equipment have been more and more highly developed, and therefore, semiconductor devices used in electronic equipment have been rapidly developed in their refinement.
Accordingly, in order to realize a high degree of integration in a dynamic RAM, a technique to use an oxide dielectric film as a capacitor dielectric film instead of a conventionally used silicon oxide or silicon nitride film has been widely studied and developed.
Also, in order to realize practical use of a nonvolatile RAM capable of operating at a lower voltage and writing/reading data at a higher speed, ferroelectric films having a spontaneous polarization characteristic are earnestly studied.
In a semiconductor memory using a ferroelectric film or a high dielectric film, in order to attain a high degree of integration of a megabit-class, stack-type memory cells are used instead of conventionally used planer-type memory cells. The most significant problem in employing the stack-type memory cells is preventing a contact face between a plug and a lower electrode of a capacitor device from being oxidized in high temperature annealing carried out in an oxygen atmosphere for crystallizing the ferroelectric film or the high dielectric film.
A conventional semiconductor device will now be described with reference to FIG. 6A.
As shown in FIG. 6A, impurity diffusion layers 11 serving as the source and the drain are formed in a semiconductor substrate 10, and a gate electrode 12 is formed on a region of the semiconductor substrate 10 sandwiched between the impurity diffusion layers 11. The impurity diffusion layers 11 and the gate electrode 12 together form a transistor.
A protection insulating film 13 is formed on the semiconductor substrate 10 so as to cover the transistor, and a plug 14 of, for example, tungsten connected to one of the impurity diffusion layers 11 is formed in the protection insulating film 13.
An adhesive layer 15 of titanium having a lower face in contact with the upper face of the plug 14 is formed on the protection insulating film 13. An oxygen barrier layer 16 of iridium oxide is formed on the adhesive layer 15, and a capacitor device composed of a capacitor lower electrode 17, a capacitor dielectric film 18 of a ferroelectric film and a capacitor upper electrode 19 is formed on the oxygen barrier layer 16. Accordingly, one of the impurity diffusion layers 11 of the transistor is electrically connected to the capacitor lower electrode 17 through the plug 14.
The oxygen barrier layer 16 has a function to prevent oxidation of the plug 14, and the adhesive layer 15 has a function to improve adhesion between the oxygen barrier layer 16 and the plug 14.
In order to crystallize the ferroelectric film used for forming the capacitor dielectric film 18, it is necessary to carry out annealing at a temperature of 600 through 800 in an oxygen atmosphere. During this annealing, a metal oxide film with high resistance is formed in the vicinity of the interface between the plug 14 and the adhesive layer 15, which disadvantageously increases the contact resistance between the plug 14 and the lower electrode 17.
Therefore, the present inventors have variously studied the cause of the formation of the metal oxide film in the vicinity of the interface between the plug 14 and the adhesive layer 15, resulting in finding the following:
FIG. 6B shows migration paths of oxygen atoms in the conventional semiconductor device, wherein denotes an oxygen atom and an arrow denotes a migration path of the oxygen atom.
In the annealing for crystallizing the ferroelectric film used for forming the capacitor dielectric film 18, oxygen atoms included in the oxygen atmosphere are diffused into the capacitor dielectric film 18, then migrate through a first path for passing through the capacitor lower electrode 17 and the oxygen barrier layer 16 to reach the adhesive layer 15 and through a second path for passing through a side portion of the capacitor dielectric film 18 to reach the adhesive layer 15, and finally reach the plug 14.
Although the oxygen barrier layer 16 of iridium oxide is formed on the plug 14, the oxygen barrier layer 16 cannot definitely prevent the passage of the oxygen atoms because the annealing for crystallization is carried out in an oxygen atmosphere at a high temperature.
Also, when the oxygen atoms reach the adhesive layer 15, titanium included in the adhesive layer 15 is easily oxidized into titanium oxide, and hence, the oxygen atoms reach the plug 14 after thus oxidizing the adhesive layer. The oxygen atoms having reached the plug 14 oxidize a metal, such as tungsten, included in the plug 14, which disadvantageously increases the contact resistance between the capacitor lower electrode 17 and the plug 14.
Furthermore, when the oxygen atoms reach the oxygen barrier layer 16, pin holes may be formed or the thickness is locally reduced in the oxygen barrier layer 16. Therefore, in a contact chain used for a test and including thousands or ten thousands of serially connected plugs 14, the resistance becomes abnormally high when the diameter of each plug 14 is small.
In consideration of the aforementioned conventional problems, an object of the invention is preventing contact resistance between a capacitor lower electrode and a plug from increasing by definitely preventing oxidation of the plug.
In order to achieve the object, the first semiconductor device of this invention comprises an impurity diffusion layer serving as a source or a drain of a transistor formed in a semiconductor substrate; a protection insulating film covering the transistor; a capacitor lower electrode, a capacitor dielectric film of an oxide dielectric film and a capacitor upper electrode successively formed on the protection insulating film; a plug buried in the protection insulating film for electrically connecting the impurity diffusion layer of the transistor to the capacitor lower electrode; and an oxygen barrier layer formed between the plug and the capacitor lower electrode, and the oxygen barrier layer is made from a composite nitride that is a mixture or an alloy of a first nitride having a conducting property and a second nitride having an insulating property.
In the first semiconductor device of the invention, the oxygen barrier layer formed between the plug and the capacitor lower electrode is made from the composite nitride that is a mixture or an alloy of the first nitride having a conducting property and the second nitride having an insulating property. In an oxygen atmosphere at a high temperature, the second nitride having an insulating property is more highly reactive with oxygen atoms than the first nitride having a conducting property.
Therefore, in crystallizing the capacitor dielectric film of the oxide dielectric film in an oxygen atmosphere at a high temperature, when the oxygen atoms diffuse into the oxygen barrier layer, the second nitride having an insulating property is rapidly reacted with the oxygen atoms to produce an oxide in a surface portion of the oxygen barrier layer. Since an oxide has a smaller particle size than a nitride, when the nitride is changed into the oxide, the migration paths of the oxygen atoms formed in the grain boundary of the nitride becomes complicated and elongated, which makes it difficult for the oxygen atoms to diffuse within the oxygen barrier layer. In other words, since an oxide layer for preventing diffusion of the oxygen atoms is formed in the surface portion of the oxygen barrier layer, the function of the oxygen barrier layer to prevent diffusion of the oxygen atoms can be improved.
When the nitride is changed into the oxide, the resistance of the oxygen barrier layer may be increased. The composite nitride includes, however, the first nitride having a conducting property that is comparatively less reactive with the oxygen atoms, which suppresses the increase of the resistance of the oxygen barrier layer.
Accordingly, while suppressing the increase of the resistance, the oxygen barrier layer can definitely prevent the diffusion of the oxygen atoms, resulting in definitely preventing oxidation of the plug.
In the first semiconductor device, the first nitride is preferably a nitride of at least one of titanium, tantalum, cobalt, copper and gallium, and the second nitride is preferably a nitride of at least one of aluminum, silicon, chromium, iron, zirconium and hafnium.
When the nitride of aluminum, silicon, chromium, iron, zirconium or hafnium is brought into contact with oxygen atoms at a high temperature, the nitride rapidly changes into an oxide, so as to prevent the diffusion of the oxygen atoms. Therefore, the function of the oxygen barrier layer to prevent the diffusion of the oxygen atoms can be definitely improved. Furthermore, since the nitride of titanium, tantalum, cobalt, copper or gallium is difficult to oxidize even at a high temperature and is less degraded in its conducting property even when oxidized, the increase of the resistance of the oxygen barrier layer can be suppressed.
The first semiconductor device preferably further comprises an upper oxygen barrier layer formed between the oxygen barrier layer and the capacitor lower electrode and made from a metal that has a conducting property when it is oxidized.
When the upper oxygen barrier layer of the metal that has a conducting property even when oxidized is thus formed on the oxygen barrier layer, two oxygen barrier layers are present on the plug. Therefore, the function to prevent the diffusion of the oxygen atoms can be further improved and the increase of the resistance can be prevented.
In this case, the metal is preferably at least one of iridium, ruthenium, rhenium, osmium, rhodium, platinum and gold.
Thus, when the oxygen atoms diffuse into the upper oxygen barrier layer, a metal oxide layer that prevents the migration of the oxygen atoms and does not have very high resistance is formed in a surface portion of the upper oxygen barrier layer. Therefore, the diffusion of the oxygen atoms can be more effectively prevented.
The first semiconductor device preferably further comprises an upper oxygen barrier layer formed between the oxygen barrier layer and the capacitor lower electrode and made from a metal oxide having a conducting property.
When the upper oxygen barrier layer of the metal oxide having a conducting property is thus formed on the oxygen barrier layer, two oxygen barrier layers are present on the plug. Therefore, the function to prevent the diffusion of the oxygen atoms can be further improved and the increase of the resistance can be prevented.
In this case, the metal oxide is preferably at least one of an iridium oxide, a ruthenium oxide, a rhenium oxide, an osmium oxide and a rhodium oxide.
Thus, the oxygen atoms diffusing through the upper oxygen barrier layer are prevented from migrating by the metal oxide, and hence, the diffusion of the oxygen atoms can be more effectively prevented.
The first semiconductor device preferably further comprises an upper oxygen barrier layer of a multi-layer structure composed of a first metal layer of a metal that has a conducting property when it is oxidized and a second metal layer of a metal oxide having a conducting property.
Thus, even when a defect is caused in one of the first metal layer and the second metal layer, the other metal layer can prevent the passage of the oxygen atoms. Therefore, the diffusion of the oxygen atoms can be definitely prevented.
The second semiconductor device of this invention comprises an impurity diffusion layer serving as a source or a drain of a transistor formed in a semiconductor substrate; a first protection insulating film covering the transistor; a plug buried in the first protection insulating film and having a lower end electrically connected to the impurity diffusion layer of the transistor; an oxygen barrier layer formed on the first protection insulating film and having a lower face connected to an upper end of the plug; a capacitor lower electrode formed on the oxygen barrier layer; a second protection insulating film formed on the first protection insulating film to cover peripheral faces of the oxygen barrier layer and the capacitor lower electrode and having an upper face placed at substantially the same level as an upper face of the capacitor lower electrode; a capacitor dielectric film made from an oxide dielectric film formed on the capacitor lower electrode and the second protection insulating film and having a plane shape larger than a plane shape of the capacitor lower electrode; and a capacitor upper electrode formed on the capacitor dielectric film.
In the second semiconductor device of this invention, the second protection insulating film is formed so as to cover the peripheral face of the oxygen barrier layer. Therefore, in crystallizing the capacitor dielectric film of the oxide dielectric film in an oxygen atmosphere at a high temperature, oxygen atoms included in the oxygen atmosphere pass through the second protection insulating film before reaching the oxygen barrier layer, and hence, the number of oxygen atoms that can reach the oxygen barrier layer can be reduced. Also, since the capacitor dielectric film has a plane shape larger than that of the capacitor lower electrode, the oxygen atoms included in the oxygen atmosphere migrate by a long distance, namely, take a roundabout way, within the second protection insulating film before reaching the oxygen barrier layer. Therefore, the number of oxygen atoms that can reach the oxygen barrier layer can be further reduced.
Accordingly, the number of oxygen atoms that diffuse through the oxygen barrier layer to reach the plug can be largely reduced, resulting in definitely preventing the oxidation of the plug.
In the second semiconductor device, the oxygen barrier layer is preferably made from a composite nitride that is a mixture or an alloy of a first nitride having a conducting property and a second nitride having an insulating property.
Thus, when the oxygen atoms pass through the second protection insulating film to reach the oxygen barrier layer, the second nitride having an insulating property is changed into an oxide in a surface portion of the oxygen barrier layer. Therefore, the oxygen atoms are difficult to diffuse into the oxygen barrier layer, resulting in largely improving the function of the oxygen barrier layer to prevent the diffusion of the oxygen atoms.
The second semiconductor device preferably further comprises an upper oxygen barrier layer formed between the oxygen barrier layer and the capacitor lower electrode and made from a metal that has a conducting property when it is oxidized.
Thus, two oxygen barrier layers are present on the plug, and hence, the function to prevent the diffusion of the oxygen atoms can be further improved, and the increase of the resistance can be prevented.
The second semiconductor device preferably further comprises an upper oxygen barrier layer formed between the oxygen barrier layer and the capacitor lower electrode and made from a metal oxide having a conducting property.
Thus, two oxygen barrier layers are present on the plug, and hence, the function to prevent the diffusion of the oxygen atoms can be further improved, and the increase of the resistance can be prevented.
The method for fabricating a semiconductor device of this invention comprises the steps of forming an impurity diffusion layer serving as a source or a drain of a transistor in a semiconductor substrate; forming a first protection insulating film covering the transistor; burying, in the first protection insulating film, a plug having a lower end electrically connected to the impurity diffusion layer of the transistor; forming, on the first protection insulating film, an oxygen barrier layer having a lower face connected to an upper end of the plug; forming a capacitor lower electrode on the oxygen barrier layer; forming, on the first protection insulating film, a second protection insulating film covering the oxygen barrier layer and the capacitor lower electrode, and planarizing the second protection insulating film, whereby placing an upper face of the second protection insulating film at substantially the same level as an upper face of the capacitor lower electrode; forming a capacitor dielectric film having a plane shape larger than a plane shape of the capacitor lower electrode by depositing an oxide dielectric film on the capacitor lower electrode and the second protection insulating film and patterning the oxide dielectric film; and forming a capacitor upper electrode on the capacitor dielectric film.
In the method for fabricating a semiconductor device of this invention, after forming the second protection insulating film so as to cover the oxygen barrier layer and the capacitor lower electrode, the second protection insulating film is planarized so as to place the upper face of the second protection insulating film at substantially the same level as the upper face of the capacitor lower electrode. Therefore, the capacitor dielectric film of the oxide dielectric film is crystallized in an oxygen atmosphere at a high temperature with the peripheral face of the oxygen barrier layer covered with the second protection insulating film, and hence, oxygen atoms included in the oxygen atmosphere pass through the second protection insulating film before reaching the oxygen barrier layer. Also, since the capacitor dielectric film has a plane shape larger than that of the capacitor lower electrode, the oxygen atoms included in the oxygen atmosphere migrate by a long distance within the second protection insulating film before reaching the oxygen barrier layer, and hence, the number of oxygen atoms that can reach the oxygen barrier layer can be largely reduced.
Accordingly, the number of oxygen atoms that diffuse through the oxygen barrier layer to reach the plug can be largely reduced, resulting in definitely preventing oxidation of the plug.
In the method for fabricating a semiconductor device, the oxygen barrier layer is preferably made from a composite nitride that is a mixture or an alloy of a first nitride having a conducting property and a second nitride having an insulating property.
Thus, when the oxygen atoms pass through the second protection insulating film to reach the oxygen barrier layer, the second nitride having an insulating property is changed into an oxide in a surface portion of the oxygen barrier layer. Therefore, the oxygen atoms are difficult to diffuse into the oxygen barrier layer, and hence, the function of the oxygen barrier layer to prevent the diffusion of the oxygen atoms can be largely improved.
The method for fabricating a semiconductor device preferably further comprises, between the step of forming the oxygen barrier layer and the step of forming the capacitor lower electrode, a step of forming an upper oxygen barrier layer made from a metal that has a conducting property when it is oxidized.
Thus, since two oxygen barrier layers are present on the plug, the function to prevent the diffusion of the oxygen atoms can be further improved, and increase of the resistance can be prevented.
The method for fabricating a semiconductor device preferably further comprises, between the step of forming the oxygen barrier layer and the step of forming the capacitor lower electrode, a step of forming an upper oxygen barrier layer made from a metal oxide having a conducting property.
Thus, two oxygen barrier layers are present on the plug, the function to prevent the diffusion of the oxygen atoms can be further improved, and the increase of the resistance can be prevented.